<script src="./vue.js"></script>
<body>
     <div id="app">
          <input type="text" v-model="cityName"/>
     </div>
</body>
     <script>
          var vue=new Vue({
               el:"#app",
               data:{
                    cityName:'天津',
               },
               watch:{
                   cityName(newName,oldName){
                    console.log(newName,oldName)
                    }
               }
          });
     </script>
